The present invention relates to surface plasmon resonance (SPM) sensors. More particularly, the present invention relates to the field of water quality monitoring, where there is a need for sensors being able to measure a large amount of different compounds having the potential of polluting our water resources. Other possible applications are food quality monitoring, process control, biological components including human immunodeficiency virus (HIV) core protein detection and in gene expression monitoring.
Surface plasmons (SPs) are normal modes of charge density that exists at the interface between a dielectric and a metal/semiconductor. It was discovered 30 years ago that the coupling between SPs and the electromagnetic field of light is sensitive to the changes in the optical properties of the dielectric medium close to a metal surface. SPR sensors have attracted attention primarily for medical and environmental applications.
Monitoring of different analytes may be determined by an array of different molecular recognition elements (MREs), each element having a specific response to a particular analyte. The MREs can be biological, biochemical or chemical recognition elements or a combination of these elements.
MREs can for example be immobilized directly on the surface of a metal film supporting SP waves at resonance with the light (SPR metal film), e.g. through thiols binding to a gold surface.
Alternatively, the MREs can be immobilized for example through covalent binding in a suitable polymer film (e.g. hydrogel) that is a few hundred nanometers thick coating the SPR metal film. Depending on applications, various sensing schemes of MREs have been reported including antibody-antigen reactions, arrays of oligonucleotides or probes originating from cDNA libraries for DNA hybridization analysis, molecular imprinting techniques, ionic interaction with ionophores and chromo-ionophores, and electrochemical interaction where the SPR metal film acts as one of the two electrodes (the cathode or the anode). Although these MREs are very different in nature, they have the inherent property that they all make use of surface or interface sensitive bio-/chemical interactions, and these interactions can quantitatively be monitored using a SPR sensing scheme.
Since SPs propagate in the transverse magnetic mode (TM mode), optical excitation is only possible in cases where the electric field is polarized parallel to the incident plane (TM polarization) and the wave vectors of the light and the SP are matched. The wave vector kSP of the SP at the metal/dielectric interface (i.e. the interface between the metal and the sample to be measured) and at a wavelength xcex is given approximately by:                                           k            SP                    ~                                    2              ⁢              π                        λ                          ⁢                                                            ϵ                m                            ⁢                              ϵ                s                                                                    ϵ                m                            +                              ϵ                s                                                                        (        1        )            
where xcex5S and xcex5m are the real parts of the dielectric constants of the sample and the metal, respectively. The incident light cannot couple directly to SPs on smooth surfaces, since for negative values of xcex5m as is the case for metals, the wavevector for the light and the SP can never be matched. SPs can be excited either electronically, optically using a grating or optically using coupling of evanescent waves of light to a metal surface. The latter approach is often performed using the Kretschmann configuration, which consists of a thin metal film coating one face of a high index prism (npxcx9c1.4-1.7).
Light passing through the prism increases momentum and is totally reflected from the metal surface at an angle xcex8, which is greater than the critical angle between the prism and the sample. The component of the wave vector of light kev, being parallel to the metal/dielectric interface and incident on the metal surface with a wavelength xcex, is given by:                                           k            ev                    ~                                    2              ⁢              π                        λ                          ⁢                              ϵ            g                          ⁢        sin        ⁢                  xe2x80x83                ⁢        θ                            (        2        )            
where xcex5g is the dielectric constant of the prism. The parameters xcex5m and xcex5g are usually fixed and xcex5S is the dielectric constant of the sensing area to be measured and its value changes according to the analyte detection. At wave vector matching, kSP=kev, the light interacts strongly with the SP giving rise to a large decrease in the reflectivity of the light from the metal/dielectric interface. This condition characterizes the SPR and can be measured using various methods including focusing a beam with an angular band of the light covering the SPR angle, scanning the wavelength of the incident light or a combination of both methods.
A commercial SPR system from the company BlAcore is based on a Kretchmann configuration, but where the SPR metal film is disposed on a replaceable glass plate which is physically separated from a glass prism by means of a refractive index matching gel disposed in between the glass prism and the glass plate. This instrument is large and expensive and there has been much effort in the art to provide small and compact SPR sensors.
U.S. Pat. No. 5,629,774 describes a portable SPR sensor with the object of measuring analyte in a fluid. The sensor comprises a monochromatic light source, a surface plasmon resonance-sensitive device for reflecting the light and a detector based on one or more photo-detectors combined with an xe2x80x9copeningxe2x80x9d such as a pin hole. The xe2x80x9copeningxe2x80x9d defines a particular angle on the critical side of the SPR resonance minimum. Small changes in the sample produces large changes in the reflected intensity monitored by the light detector. Compared to systems employing a scanning mechanism or using a focusing light beam with an angular band of light covering the SPR angle, a disadvantage of the system described in U.S. Pat. No. 5,629,774 is related to the use of a single detector which requires a more precise alignment of the system.
In EP 0 797 090, all mirrors, the sensing layer, the photo-detector array and optionally the light source are integrated in the same house. A disadvantage of this configuration is the fact that all components have to be replaced when replacing the sensing layer.
Optional configurations have been described in EP 0 797 091, where a transparent base housing and a detachable prism-like optical housing are index matched to avoid undesirable refraction of the light rays. This is performed using index matching gel between the base housing and the optical housing or fabricating concave portions in the base housing and complementary convex portions in the optical housing at the intersections between the two housings. Both options seem to be complicated solutions for practical working SPR sensors.
It is a disadvantage of the above-mentioned systems that these systems apply an index matching gel. The gel is inconvenient to work with and it may cause problems if comes in contact with some of the optical or bio-/chemical elements.
EP 0 805 347 describes a surface plasmon sensor where the metal layer supporting the surface plasmons is positioned on a glass substrate. An incoming optical light beam is directed towards the metal layer using a first transmission grating. The incoming optical light beam is also focussed by the first transmission grating. The directed optical light beam is reflected off the metal layer and propagates towards a second transmission grating. The second transmission grating directs the transmitted beam towards a detector.
It is a disadvantage of the sensor described in EP 0 805 347 that the incoming light beam is incident under angle that differs from normal incidence.
Generally in the prior art, the SPR sensing layers, the light sources, the mirrors and the detectors have been arranged in three-dimensional configurations where at least one component is aligned at an angle close to the SPR angle (xcx9c50xc2x0-80xc2x0) compared to the other components. This implies that integration of sensors with large arrays of sensing areas cannot be readily made. Integration shall preferably be carried out laterally, which requires a planar configuration of layers or planar configurations aligned parallel to each other
Therefore, there is a need in the prior art of compact SPR sensors comprising dispensable sensor chips with a large array of sensing areas, with uncritical alignment between sensor chips and optical transducers, and with no requirements of using index matching gels.
It is an object of the present invention to provide a SPR sensor comprising a sensor chip constructed of laterally integrated arrays of planar sensor chip units (SCU)
It is a further object of the present invention to provide a SPR sensor comprising an optical transducer constructed of laterally integrated arrays of planar optical transducer units.
It is a still further object of the present invention to provide a SPR sensor comprising two separable unitsxe2x80x94a sensor unit and a transducer unit.
It is a still further object of the present invention to provide a SPR sensor with uncritical alignment between the sensor unit and the transducer unit.
It is a still further object of the present invention to provide a SPR sensor wherein the use of index matching gel is avoided.
The above-mentioned objects are complied with by providing, in a first aspect, a surface plasmon resonance sensor comprising a first unit and a second unit, said first and second units being separable, and wherein said first unit comprises:
a first housing,
a film of electrically conducting material being adapted to support surface plasmons, said film being hold by a first exterior surface part of the first housing,
optical input means positioned on a second exterior surface part of the first housing so as to receive an optical light beam from the second unit,
optical output means positioned on a third exterior surface part of the first housing so as to transmit an optical light beam to the second unit,
a first set of optical elements being adapted to direct the received optical light beam from the first unit towards the electrically conducting film,
a second set of optical elements being adapted to direct an optical light beam from the electrically conducting film towards the optical output means so as to transmit the optical light beam from the electrically conducting film to the second unit,
and wherein said second unit comprises:
a second housing,
means for emitting an optical light beam,
a first set of optical elements being adapted to prepare the emitted optical light beam,
optical output means positioned on a first exterior surface part of the second housing so as to transmit the prepared optical light beam to the first unit,
optical input means positioned on a second exterior surface part of the second housing so as to receive an optical light beam from the first unit,
detecting means being adapted to detect the optical light beam received from the first unit,
a second set of optical elements being adapted to direct the received optical light beam from the first unit towards the detecting means,
wherein the propagation directions of the optical light beams at the positions of the optical input and optical output means are essentially perpendicular to the exterior surface parts of the first and the second housing so as to avoid refraction of the optical light beams upon entry of said optical light beams into the first and second unit.
In this and the following aspects of the present invention essentially perpendicular means that the angle of incidence may be in the range xe2x88x9210xc2x0-10xc2x0, preferably in the range xe2x88x925xc2x0-5xc2x0, more preferably in the range xe2x88x922xc2x0-2xc2x0 and even more preferably in the range xe2x88x920.5xc2x0-0.5xc2x0.
The emitting means may comprise a laser source, such as a semiconductor laser diode. The light emitting means may emit light at essentially a single wavelength. Alternatively, the light emitting means may emit light at a plurality of wavelengths using e.g. a light emitting diode.
The first set of optical elements of the second unit may comprise means for collimating the emitted optical light beam. The collimating means may comprise lens means.
By collimatedxe2x80x94as mentioned here and in some of the following aspectsxe2x80x94is meant that the angular beam spread of the emitted optical light beam may be less than 10xc2x0, preferably less than 5xc2x0, more preferably less than 2xc2x0 and even more preferably less than 0.5xc2x0.
The first set of optical elements of the second unit may further comprise means for polarizing the emitted optical light beam. This polarizing means may be any kind of polarizing film, prism arrangement or voltage controlled variable retarder.
The input and output means of the first and second units may comprise antireflecting coatings.
The detecting means may comprise an array of photosensitive elements, such as a multiple photo detector array, a charge coupled device or a complementary metal oxide semiconductor image sensor. The sensor may further comprise a light shield member.
The first set of optical elements of the first unit may comprise a diffractive member, such as a diffractive grating or a holographic grating. In a similar way, the second set of optical elements of the first unit may comprise a diffractive member, such as a diffractive grating or a holographic grating. The diffractive members may be formed by reflective members. The second set of optical elements may also comprise a reflective member, such as a reflective mirror.
The electrically conducting film may comprise a metal film, such as a gold film, a silver film, an aluminum film or a titanium film. The electrically conducting film may be formed by a plurality of electrically conducting films, said plurality of films being arranged in a laterally extending pattern.
To support long range surface plasmon resonances a layer of dielectric material may be positioned between the electrically conducting film and the first exterior surface part of the first housing. In case the surface plasmon resonance sensor comprises a plurality of electrically conducting layers the sensor may further comprise a layer of dielectric material being positioned between each of the plurality of electrically conducting films and the first exterior surface part of the first housing.
The surface plasmon resonance sensor may further comprise moving means being adapted to move the first and second unit relative to each other so as to move the focus point of an optical light beam relative to an electrically conducting film. Alternatively or in addition, the surface plasmon resonance sensor may comprise moving means being adapted to move the first and second unit relative to each other so as to vary the angle of incidence of an optical light beam directed towards an electrically conducting film.
The surface plasmon resonance sensor may comprise two or more surface plasmon resonance sensors preferably being arranged in a lateral extending pattern.
In a second aspect, the present invention relates to a method of determining the bio-/chemical composition of a sample using a surface plasmon resonance sensor, said surface plasmon resonance sensor comprising a first unit and a second unit, said first and second units being separable, and wherein said first unit comprises:
a first housing,
a film of electrically conducting material being adapted to support surface plasmons, said film being hold by a first exterior surface part of the first housing,
optical input means positioned on a second exterior surface part of the first housing so as to receive an optical light beam from the second unit,
optical output means positioned on a third exterior surface part of the first housing so as to transmit an optical light beam to the second unit,
a first set of optical elements being adapted to direct the received optical light beam from the first unit towards the electrically conducting film,
a second set of optical elements being adapted to direct an optical light beam from the electrically conducting film towards the optical output means so as to transmit the optical light beam from the electrically conducting film to the second unit,
and wherein said second unit comprises:
a second housing,
means for emitting an optical light beam,
a first set of optical elements being adapted to prepare the emitted optical light beam,
optical output means positioned on a first exterior surface part of the second housing so as to transmit the prepared optical light beam to the first unit,
optical input means positioned on a second exterior surface part of the second housing so as to receive an optical light beam from the first unit,
detecting means being adapted to detect the optical light beam received from the first unit,
a second set of optical elements being adapted to direct the received optical light beam from the first unit towards the detecting means,
wherein the propagation directions of the optical light beams at the positions of the optical input and optical output means are essentially perpendicular to the exterior surface parts of the first and the second housing so as to avoid refraction of the optical light beams upon entry of said optical light beams into the first and second unit.
In a third aspect, the present invention relates to a surface plasmon resonance sensor comprising a first unit, said first unit comprising:
a first housing,
a layer of electrically conducting material being adapted to support surface plasmons, said layer being held by a first exterior surface part of the first housing,
optical input means positioned on a second exterior surface part of the first housing, said optical input means being adapted to receive an optical light beam,
optical output means positioned on a third exterior surface part of the first housing, said optical output means being adapted to transmit an optical light beam,
a first diffractive optical element being adapted to direct the received optical light beam towards the electrically conducting layer,
a second diffractive optical element being adapted to direct a reflected optical light beam from the electrically conducting layer towards the optical output means,
wherein the propagation directions of the optical light beams at the positions of the optical input and optical output means are essentially perpendicular to the exterior surface parts of the first housing so as to avoid refraction of the optical light beams at the positions of the optical input and optical output means.
The surface plasmon resonance sensor according to the third aspect may further comprise a second unit, said second unit comprising:
a second housing,
means for emitting an optical light beam,
a set of optical elements being adapted to prepare the emitted optical light beam,
optical output means positioned on a first exterior surface part of the second housing, said optical output means being adapted to transmit the prepared optical light beam to the first unit,
optical input means positioned on a second exterior surface part of the second housing, said optical input means being adapted to receive an optical light beam from the first unit,
detecting means being adapted to detect the received optical light beam from the first unit,
wherein the propagation directions of the optical light beams at the positions of the optical input and optical output means are essentially perpendicular to the exterior surface parts of the second housing so as to avoid refraction of the optical light beams at the positions of the optical input and optical output means.
The second unit may further comprise an optical element being adapted to direct the received optical light beam from the first unit towards the detecting means.
The light emitting means may comprise light sources as described in relation to the first aspect of the present invention. The set of optical elements of the second unit may comprise collimating and/or polarizing means as described in accordance with the first aspect of the present invention.
The input and output means of the first and second units may be coated with an antireflecting coating.
As with the first aspect of the present invention, the detecting means may comprise an array of photosensitive elements, such as a multiple photo detector array, a charge coupled device or a complementary metal oxide semiconductor image sensor. The first and second diffractive optical element of the first unit may comprise an optical grating, such as a reflective holographic grating.
In a fourth aspect, the present invention relates to a surface plasmon resonance sensor comprising:
a transparent member,
a layer of electrically conducting material being adapted to support surface plasmons, said layer being held by an exterior surface part of the member,
a first optical grating being held by a first exterior surface part of the member and being adapted to direct a received optical light beam towards the electrically conducting layer, wherein the propagation direction of the received optical light beam at the position of the first optical grating is essentially perpendicular to the first exterior surface part of the member and wherein the received optical light beam is collimated, and
a second optical grating being held by a second exterior surface part of the member and being adapted to receive an optical light beam from the electrically conducting layer and being adapted to re-emit the optical light beam received from the electrically conducting layer, wherein the propagation direction of the re-emitted optical light beam at the position of the second optical grating is essentially perpendicular to the second exterior surface part of the member and wherein the re-emitted optical light beam is collimated.
The surface plasmon resonance sensor according to the fourth aspect may further comprise
means for emitting an optical light beam,
a set of optical elements being adapted to prepare the emitted optical light beam, and
means for detecting the re-emitted optical light beam.
Even further, the surface plasmon resonance sensor according to the fourth aspect may comprise an optical element being adapted to direct the re-emitted optical light beam towards the detecting means.
The light emitting means may comprise light sources as described in relation to the first and third aspect of the present invention. The set of optical elements of the second unit may comprise collimating and/or polarizing means.
The detecting means may comprise an array of photosensitive elements, such as a multiple photo detector array, a charge coupled device or a complementary metal oxide semiconductor image sensor.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.